1. Field of the Invention
The present invention relates generally to the transmission of optical fiber signal communications and, more particularly, to an optical attenuator which employs a high-loss optical waveguide section.
2. Description of the Prior Art
The advantages of using optical transmission systems for communications are well recognized. Optical waveguides comprising dielectric fibers having a substantially transparent core coaxially surrounded by a cladding material of lower dielectric index may be used to guide and transmit optical signals over long distances in optical communications systems. Generally, great care is taken to minimize light losses due to absorption and scattering along the length of the filament, so that light applied to one end of the optical filamentary material is efficiently transmitted to the opposite end of the material. For this reason, low attenuation optical waveguides are commonly formed from fibers doped with rare earth elements. There are many situations, however, in which it is necessary to utilize optical attenuator devices to reduce the amount of power present in the optical signal.
Two output characteristics are usually described for an optical transmission system: the transmission rate, e.g. in Mbit/s as a measure of the amount of transmitted data, and the system range, which indicates the maximum attenuation that may be placed between emitter and receiver in order to assume a certain minimum quality of transmission. However, further information is needed for practical use. A receiver is only able to function optimally within a certain range of the optical input level. Too low a radiation capacity, as well as too high a level, can impair the transmission quality. The path attenuation in optical transmission systems is a function of fiber length and the fiber attenuation coefficient. In addition, emitter output and receiver sensitivity have tolerances and may exhibit aging. For these reasons, an attenuation device for adapting the path attenuation to the receiver's optimum function range is needed.
For example, disclosed in U.S. Pat. No. 5,187,610 entitled LOW NOISE, OPTICAL AMPLIFIER HAVING POST-AMPLIFICATION LOSS, issued to Habbab, et. al. and assigned to the assignee herein, AT&T, is a technique for improving the noise performance of an optical amplifier while concurrently meeting the amplifier design criteria for output signal power, amplifier gain, and compression. These benefits are obtained by combining an optical amplifier element with a post-amplification optical attenuator/loss element and pumping the optical amplifier to produce a higher gain and, therefore, a larger output signal power which is substantially compensated by the post-amplifier loss element. Compensation by the loss element allows the combination of elements to produce an output signal power which meets the design criterion.
Habbab et al. provide several examples of conventional passive optical attenuation devices capable of serving as the means for introducing a post-amplifier loss in the inventive system. The loss introducing techniques suggested in that patent include the use of a fiber-to-fiber coupler having an intentional misalignment between the two fibers to cause the desired amount of loss or providing curvature or bending of an optical fiber or dielectric waveguide to subject the lightwave signal to controllable amounts of loss as a function of the radius of the curve or bend. In each of these loss introducing techniques, precise adjustments to the fiber gap or curvature are necessary in order to achieve the requisite amount of attenuation.
Fiber optic attenuating devices are also employed as terminations for the ends of unused optical fibers of devices such as star couplers to eliminate deleterious reflections. As will be readily understood by those skilled in the art, an optical star coupler is a device which comprises a plurality of input optical fibers, a coupling region, and a plurality of output optical fibers. An optical star coupler typically operates to transmit a fraction of the optical power received at each input fiber to all output fibers and is particularly useful for implementing an optical bus which enables a plurality of terminals to communicate with one another.
A typical, off-the-shelf star coupler is an 8.times.8 device, i.e., it comprises eight input fibers and eight output fibers. However, in a typical application not all of the input fibers receive optical signals and not all of the output fibers are connected to other fibers for transmitting optical signals to remote locations. For example, to provide a 4.times.4 coupler, four of the eight input fibers are not utilized and four of the eight output fibers are not utilized. These unused fibers conveying output signals give rise to undesired reflections that result from the fiber-air index of refraction mismatch at the ends of the unused fibers. Typically, the index of refraction mismatch at a glass fiber-air interface results in a reflection of four percent of the optical signal.
Thus, in a 4.times.4 coupler formed by an 8.times.8 star coupler having four unused input fibers and four unused output fibers, the optical signal arriving on each of the used or connected input fibers is distributed by the coupling region to all eight output fibers. The radiation distributed to the used output fibers is transmitted via connector assemblies to other fibers for transmission to remote locations. At the ends of the four unused output fibers, reflections take place. The reflected radiation is then distributed by the coupling region to all the input fibers where reflection again takes place at the glass-air interfaces at the ends of the unused input fibers. This reflected radiation is then, in turn, transmitted by the coupling region back to the output fibers, and so on.
Because glass-air interfaces at the ends of unused fibers cause multiple reflections in a device such as an optical coupler, a variety of reflection-less terminating devices have been proposed. For example, in U.S. Pat. No. 4,998,795 entitled "REFLECTION-LESS TERMINATOR" and issued to Bowen et al., a terminator comprising a length of optical fiber is described. The front end of the fiber is attached to a ferrule for mating with a connector plug attached to the end of the fiber to be terminated. The rear end of the fiber is crushed at an angle and inserted into an index matching opaque adhesive material. Because there are substantially no reflections at the fiber-adhesive material interface, substantially all of the radiation propagating in the fiber length is transmitted into the opaque index matching adhesive where this radiation is absorbed. While the device taught by Bowen et al. does appear to achieve a substantially reflection-less termination, its complex structure requires several labor-intensive processing steps and may degrade in performance over time.
In view of the above, it would be advantageous to provide a passive optical signal attenuating element which may be flexibly configured to provide a controlled degree of attenuation such that it may be inserted at any point in an optical path to introduce a desired amount of loss or be utilized to provide a substantially reflection-less termination for an optical fiber.